The present invention relates to a thermoplastic resin composition of .alpha.-methylstyrene-rich copolymer, vinyl graft copolymer, and polycarbonate, said composition being superior in thermal deformation resistance, impact resistance, and flowability in hot molding.
ABS resin (acrylonitrile-butadiene-styrene graft copolymer) has been in general use as an inexpensive material which is superior in mechanical properties and flowability in hot molding. However, ABS resin is not used in the areas where thermal deformation resistance higher than 90.degree. C. is required. In addition, it is not used for making articles which require high impact resistance, toughness, and thermal deformation resistance.
On the other hand, polycarbonate has been used as a material which is superior in impact resistance, toughness, and thermal deformation resistance. However, it has drawbacks that the flowability in hot molding is poor, the price is high, and thick articles molded from it are brittle. Efforts have been made to remedy these drawbacks. For instance, there are shown the blending of ABS resin into polycarbonate in Japanese Patent Publication No. 15225/1963, the blending of styrene and methacrylate into a styrene-butadiene copolymer and the blending of a copolymer grafted with styrene, methyl methacrylate, and acrylonitrile into a styrene-butadiene copolymer in Japanese Patent Publication No. 71/1964, the blending of polystyrene in Japanese Patent Publication No. 6295/1968, and the blending of methyl methacrylate resin in Japanese Patent Publication No. 48500/1971. The compositions based on these known technologies, which are intended mainly to improve the flowability of polycarbonate in hot molding, do not necessarily possess preferable properties over a broad blending range. They lack either or both of mechanical strength (particularly impact strength and toughness) and thermal deformation resistance. Because of their unsatisfactory results, they are used only in limited areas.
There is disclosed in Japanese Patent Publication No. 504/1976 a technology for blending polycarbonate with a vinyl thermoplastic resin which is a mixture of a vinyl graft copolymer and a vinyl linear polymer consisting mainly of .alpha.-methylstyrene. The composition according to this known technology has comparatively good thermal deformation resistance and both high impact resistance and toughness. Nevertheless, it is necessary to increase the blend ratio of polycarbonate for specific application areas where a high level of thermal deformation resistance is required. This leads to high cost. In addition, it has been pointed out that such a blend composition suffers from a disadvantage. That is, if the content of .alpha.-methylstyrene in the vinyl linear polymer consisting mainly of .alpha.-methylstyrene is increased above the limit defined in the claim, the composition does not improve any longer in thermal resistance and mechanical properties but decomposes in molding.